The plating of copper from acid solutions is well known, with numerous industrial applications. In most applications the articles to be plated are suspended in the electrolyte, a technique hereafter called rack plating. Known applications include decorative finishes for household and automotive goods, electroforming, and production of printing cylinders. Other applications will be well known to those with knowledge of the electroplating industry.
The electroplating of parts normally takes place in a suitable tank containing an electrolyte into which the article to be plated is partially or wholly immersed. The article to be electroplated is suitably pre-treated prior to deposition of copper in order to provide a surface that will be receptive to the copper coating and give an adherent deposit. Copper deposition is effected by making the article to be plated the cathode in a circuit, and by passing a direct electric current through the article and electrolyte with suitable anodes completing the circuit with a power supply. The tanks are normally fitted with filtration and temperature control equipment to provide good process control. Solution agitation equipment such as air or solution movement may be utilised if desired.
The base composition of the electrolyte typically comprises 50-250 g/l of copper sulphate pentahydrate, 20-150 ml/l of concentrated sulphuric acid, optionally about 20-200 mg/l of chloride ion, and optionally proprietary additives. Baths typically used for electronics applications use low copper sulphate and high sulphuric acid concentrations, whilst baths typically used for electroforming, decorative applications or printing cylinder production generally use high copper sulphate and low sulphuric acid concentrations.
The use of pulse reverse plating techniques to deposit copper from acidic solutions is well-known within the electronics industry, for plating copper from acidic solutions onto printed circuit boards and other substrates. U.S. Pat. No. 6,319,384, to Taylor et al., the subject matter of which is herein incorporated by reference in its entirely, discloses a method for the electrodeposition of copper onto a semiconductor substrate, wherein the acidic copper plating bath is substantially devoid of brighteners and and/or levellers.
The basic chemistry of the additives used for electronics applications, and their performance under pulse reverse current plating conditions as compared to direct current conditions is explained by T. Pearson, “Effect of Pulsed Current On The Electrodeposition of Chromium and Copper”, PhD thesis, Aston University, United Kingdom, 1989, the subject matter of which is herein incorporated by reference in it is entirety. The additives broadly comprise a sulphopropyl sulphide and a polyalkylene glycol that operate in conjunction with chloride ion. Generally these baths for electronics applications produce matt copper deposits that are relatively soft, in the order of 100 to 120 HV50 (Vickers Hardness measured with a 50 g weight).
A recent U.S. application Ser. No. 10/274,634 describes the use of pulse reverse plating with acidic copper electrolytes for decorative copper applications such as plating on plastics for automobile or sanitary applications, or plating on alloy automobile wheels. The pulse plating process provides for improved distribution of the copper deposit across the substrate. Such baths also contain a levelling agent to provide for a bright and lustrous copper deposit.
A recent U.S. application No. 2002/0079228 (lapsed), attributed to Robert Smith, describes an apparatus and method for electroplating of gravure printing cylinders. The method employs the application of pulse reverse plating to a bath based upon copper sulphate, sulphuric acid and chloride ion with no additives to minimize surface pitting and nodules.
The production of printing cylinders requires a copper deposit of specific hardness and additives are generally used to control this. These additives are typically (but are not limited to) sulphur compounds added to the electrolyte, normally in the concentration range of 1-100 mg/l. Some printing cylinders require copper deposits to have a hardness of about 210 HV (e.g. rotogravure cylinders), whilst cylinders for other applications may require hardness of about 240 HV (embossing) or 190 HV (etching). Also it is necessary that the hardness remains stable over an extended period of time. Additive packages for use in decorative applications frequently produce deposits with hardness in the order of 200 HV50 that self-anneal and become soft (120-150 HV50) over a period of 1-2 weeks.
Electroplating chromium from hexavalent plating baths with pulsed current has been found to produce differences in hardness (Miller & Pan, Plating and Surface Finishing 1992 page 49). Sutter et al reported differences in hardness of nickel deposits by use of pulse current (Interfinish 1984), as did Kendrick (Trans. I.M.F. Vol 44 p 78-83) and Crossley et al (Trans. I.M.F. Vol 45 p 68-83). Pearson has also reported differences in the hardness of chromium deposited from hexavalent chromium solutions (T. Pearson, “Effect of Pulsed Current On The Electrodeposition of Chromium and Copper”, PhD thesis, Aston University, United Kingdom, 1989), but found little difference in the hardness of copper deposits when plated by pulse reverse current instead of DC current. Hardnesses in the range of 100 to 120 HV50 were reported when using electrolyte formulations typically used for electronic applications.
The current application discloses the invention that variation of current profile can be used to control the hardness of a copper deposit. This is of particular advantage to the plater of printing cylinders as the same electrolyte can be used to produce copper deposits of different hardness, thereby improving the operational adaptability of a plant. Additionally it may be possible to reduce the number of electroplating tanks required in the production plant, or alternatively to increase production capacity. However the inventors understand that the application of variable current profile to provide for hardness control of the copper deposit is not limited to the production of printing cylinders, and may also be used for other electroplating applications.